Dissolved organic matter (DOM) represents the most labile part of soil organic matter (SOM) and actively participates in biogeochemical processes. However, how ecosystem properties, such as clay minerals and vegetation types, affect molecular composition of DOM is not fully understood. In this study, we characterized the molecular composition of water-extracted OM from topsoils under grass, pine or juniper across a semiarid chronosequence using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The clay content increases with increasing soil age of the chronosequence. Results show that for each vegetation type, lignin-like compounds are dominant (60-70%) in DOM and their proportions increase with increasing clay content, while the proportions of condensed aromatics (16-32%) decreased with increasing clay contents. However, the proportions of lipid-like (2-6%) and protein-like (3-12%) compounds and others (including tannin, amino sugar, carbohydrate, and unsaturated hydrocarbon with their total proportions &lt; 3%) fluctuate with clay content. The positive relationship between lignin-like compounds and clay contents suggests that clays stabilize the compounds. Furthermore, the average molecular weight of DOM declines with increasing clay content, probably because of preferential adsorption of compounds with high molecular weights by clays. We further examined the effect of vegetation types on DOM composition. At each site, the soils under grass contain more lipid-like, protein-like, and amino-sugar compounds but less lignin-like and condensed aromatic compounds than the soils under both juniper and pine trees, consistent with that high quality litter (from grass) decomposes faster than low quality litter (from trees). Our study suggests that both clay mineral contents and vegetation types can affect DOM composition in semiarid ecosystems.

Effects of clay minerals and vegetation type on the composition of dissolved organic matter in soils of a semi-arid environment

ACS Fall 2021

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